vacuum

The Wards are open-topped, with skyscrapers rising from the superstructure. Towers are sealed against vacuum, as the breathable atmosphere envelope is only maintained to a height of about seven meters. The atmosphere is contained by the centrifugal force of rotation and a "membrane" of dense, colorless sulphur hexafluoride gas, held in place by carefully managed mass effect fields.

a vacuum of 26 inches of mercury, or 13 pounds per square inch

Henrietta soon found a terrible vacuum left, by the letters in which she used to pour forth every feeling and thought to her uncle.